Brake Accumulator 737 NG

[hwood]

The way I understand it, if the brake accumulator (ba) loses it's 1000 psi pre- charge the brake pressure gauge will read zero but if normal hydraulic pressure is available braking is unaffected. What I don't understand is why with a good pre-charge and normal hydraulics does the accumulator gauge read approx 3000 psi (normal hyd pressure) ? I would think then that with loss of pre-charge and normal hydraulic pressure the brake pressure gauge would read 3000 psi. Any ideas

[BOAC]

Think of it as a sealed plate in the cylinder: the pressure sensor is below the plate.

Pre-charge 1000psi, sensor reads 1000 psi

Normal Ops 3000 psi ABOVE the plate compressess the pre-charge gas to 3000psi and sensor reads 3000psi

Lose the pre-charge - the plate moves to the bottom of the accumulator as soon as pressure is applied above.. No matter what pressure is put in above, sensor reads 0 - no gas and no space to pressurise the gas.

[Fullblast]

If you have 2 pumps each one delivering 200 Liters/minute at 3000 psi and put them on the same output line you will have 400 liters/minute at 3000 psi; basically you double the quantity but the pressure stays the same. In the particular case the precharge is the minimum you need to stop the plane.
If you have a chance, after checked chocks on, switch off the hydraulic pumps and start pumping the brakes, you will have around 8 to 10 available brakings with 3000 psi available, but once you reach 1000 psi on the gauge you won't have more than 2/3 left.

[itsresidualmate]

An accumulator is a container with a floating piston inside; one end of the accumulator container is connected to system hydraulics, the other end filled with nitrogen with a charging valve.

The only time you can see nitrogen pressure is when there is no opposing hydraulic pressure, i.e no system pumps running and pressure part of the system vented to return. When this is the case you'll see true nitrogen pressure.

If accumulator pressure is down from a normal of say 1000psi to 500 psi for instance, the accumulator gauge attached to the accumulator will show 500psi and the system and brake gauges will show zero.
Start the pumps and system pressure will show on the system gauges and brake gauges. The hydraulic fluid on the hyd side of the floating accumulator piston will compress the 500 psi of nitrogen by moving the floating piston and you'll also see system pressure on the accumulator gauge. But you won't have as much volume of stored pressure. You can probably get down to a couple of hundred psi of nitrogen and when the pumps are on you'll see system pressure on the accumulator gauge. If you were to open the nitrogen charging valve when the pumps are on, you could bleed off the remaining nitrogen until the floating piston is pushed onto its stops, then the accumulator gauge will show true nitrogen pressure and eventually 0psi as its opened to atmosphere.